A free program for Android, by Brian J Owings.

Are you looking for a powerful modular synthesizer? You’ve come to the right place. ModSynth is a free app allowing you to design and create your musical instruments using a graphical editor. In the app, you can connect any number of modules to design a complex instrument. Each module is fully controllable and you can adjust the settings of each module while playing the instrument to get a desired sound. You can save multiple instruments to help you get started. You can also record the sounds you create to a WAV file to save them.

The app is fully compatible with external MIDI devices such as keyboards or audio-based software such as DAWs. It has built-in support for low-latency MIDI devices. You can map the controls of external MIDI devices to the modules.